7 - Neuromuscular and spinal cord Flashcards
What two inputs can the membrane potential of post-synaptic neurone be altered by?
Excitatory post synaptic potential (EPSP) - made less negative - depolarisation
Inhibitory post synaptic potential (IPSP) - made more negative - hyperpolarisation
What is the neurotransmitter for the NMJ?
ACh
Describe the process that takes place at the NMJ that triggers the action potential in the muscle fibres?
action potential arrives —–> Ca2+ influx —–> ACh release —–> binds to receptors on motor end plates —–> propagates action potential (Na+ channels open) —–> actin and myosin (muscle contraction)
What are mEPPS?
mini end plate potentials
at rest, individual vesicles in the presynaptic membrane release ACh at a very slow rate leading to small fluctuations in the post-synaptic membrane potential
What are the alpha motor neurones?
the lower motor neurones of the brainstem and spinal chord
What kind of muscle fibres do alpha motor neurones innervate?
extrafusal muscle fibres (standard skeletal muscles that contract)
What is the difference between intrafusal and extrafusal muscle fibres?
Intrafusal – these are skeletal muscle fibres that serve as sensory organs (proprioceptors) that detect the amount and rate of change of length of a muscle
Extrafusal – standard skeletal muscle fibres that are innervated by alpha motor neurones and generate tension by contracting, thereby allowing for skeletal muscle movement
What is a motor neurone pool?
Collection of lower (alpha) motor neurones that innervate a single muscle
Define motor unit
A single motor neurone together with all the muscle fibres that it innervates - it is the smallest functional unit that can generate force.
Describe the organisation of cell bodies in the ventral horn?
- those that innervate flexor muscles are in the posterior part
- those that innervate extensor muscles are in the anterior part
- those that innervate proximal muscles are more medial and those that innervate distal muscles are more lateral
What is an important rule to remember regarding the connections between alpha motor neurones and muscle fibres?
One motor neurone can innervate several muscle fibres
But every muscle fibre can only be innervated by one motor neurone
Under what conditions can this rule be broken?
(That:
one motor neurone can innervate several muscle fibres, but every muscle fibre can only be innervated by one motor neurone)
Under pathological conditions (e.g. severed nerve), the axonal regeneration can result in the innervation of muscle fibres that are already innervated
Describe and explain the difference in innervation ratio across different muscles in the body using examples.
Muscles that require very fine control (e.g. extrinsic eye muscles) have a low innervation ratio (few fibres innervated by a single neurone)
Muscle that are required to generate a lot of power have a high innervation ratio because when the motor unit fires, it will cause the contraction of a large mass of muscle fibres thus generating power (e.g. quadriceps)
What are the 3 types of motor unit?
Slow (S)
Fast (FR/type IIA and FF/type IIB) - fatigues easily and resistant to fatigue
Describe the properties of the different types of motor units when they are stimulate?
slow - generate little force, but over a long period of time (e.g. standing)
fast - generate more force but cannot sustain it for very long
Describe the structural and functional differences between slow and fast twitch muscle fibres.
Slow fibres have: • Smallest diameter cell bodies • Small dendritic trees • Thinnest axons • Slowest conduction velocity Fast fibres have: • Larger diameter cell bodies • Large dendritic cells • Thicker axons • Faster conduction velocity
What are the two mechanisms by which the brain regulates the force that a single muscle can produce?
RECRUITMENT:
recruiting more motor units for the muscle contraction using the ‘size principle’
RATE CODING:
increasing the frequency of action potentials travelling down the nerves to the muscle fibres
What is the ‘size principle’ that governs recruitment?
Describe the order of recruitment of motor units with increasing force generation.
Smaller units are recruited first, which are generally slow fibres
Slow -> Fast Fatigue-Resistant -> Fast Fatiguable
What are neurotrophic factors?
They are a type of growth factor produced within the nerves and are transported throughout the nerve to maintain the nerves integrity and function.
They are a type of growth factor that prevents neuronal death and promotes the growth of neurones after injury.
describe the regeneration of neurones in the nervous system
CNS neurones don’t regenerate after injury
neurones in a peripheral environment allow for axonal regeneration
When a fast nerve is transplanted onto a slow muscle fibre, the fibre becomes fast. What does this show?
This shows that the function of the muscle fibre is very much determined by the type of nerve that innervates it.
The action potentials can’t be the only thing being delivered to the muscles by the nerves.
How easy is it to switch from one motor unit type to another?
Type 2B to Type 2A can happen with training
There is usually no way of changing from type 2 to type 1 or vice versa except in the case of severe deconditioning e.g. zero gravity or spinal injury
How does muscle composition change with ageing?
What is the generic name for this?
Ageing is associated with a loss of type 1 and 2 fibres (preferentially loss of type 2 fibres)
So a large proportion of muscle fibres in ages muscle are type 1
This loss of muscle is called sarcopenia
What tract is responsible for voluntary movements?
Pyramidal/Corticospinal tract
